Photographic bath



Reiadutd Oct. .30, 1934 moroesarmc aa'rn Felix a. sum, New York, N. 1., asalgnor to Eastman Kodak Company, Rochester, N. Y., a

corporation of New York No Drawing. Original No. 1,411,687, dated April 4, 1922, Serial No. 460,995, April 13, 1921. Application for reissue July 13', 1934, Serial No.

6 Claim.

This invention relates to photographic baths and more particularly to acid fixing baths.

It is well known that solutions of sodium thiosulfate, used for the removal from photographic emulsions of undeveloped silver salts, should be acidified. Alum is generally added to the fixing bath for the purpose of hardening the g'elatine, and if the bath is or becomes alkaline, or too weakly acid, .there is a tendency for aluminum 10 compounds to precipitate alumina or aluminum salts. 0n the other hand, it the bath is too acid, sulfur is precipitated from the sodium thiosulfate. It is necessary, however, to have enough acid present to neutralize the considerable u amounts of alkali brought over from the developer.

In an attempt to satisfy these requirements, organic acids are generally used because they are but slightly dissociated in solution and hence in render the bath sufllciently acid, while since the total amount of alkali which can be neutralized depends on the total amount of acid present, the undissociated acid 'acts as a reserve from which the acidity, that is the hydrogen ion concentration, of the bath is replenished ormaintained with gradually decreasing strength until the acidity of the bath becomes less than is desirab e.

- The amounts of these organic acids that can be added, however, are not sumcient to maintain the acidity of the bath within the desired limits during the life of the bath and it is necessary either to add acid or discard the bath.

I have discovered that the acidity of such 'a a bath may be maintained within the desired limits for the full life of the bath by the' bufler action" of certain substances in solution. 13y the full life of the bath I mean until the sodium thiosulfate is exhausted. By "butler action" is o meant the ability ofa solution to resist change in its hydrogen ion concentration through the addition or loss of acid or alkali. The unit by which hydrogen ion concentration is measured is defined as represented by the symbol pHorPs. I have here adopted the former symbol consistently with its use in the Journal of Biological Chemistry. Full explanation of the theory underlying the "buifer action" referred to above is contained in the literature, and it is unnecessary. here to review it. Reference is made particularly to the comparatively simple explanation found in (The Determination of'Hydrogen Ions. by Clark, published 1920, by the Williams and Wilkins Com-- pany, Baltimore, Maryland, and to the extensive 5 bibliography contained therein.

For a neutral solution, the expression pH=7 is approximately correct, and it has been found that the fixing bath should have an acidity r hydrogen ion concentration not greater than pH=4 and not less than pH=6.5 or,- preferably, pI-I=6. It is possible by applying known ionization formula: to compute the required amount of" suitable butters, that is substances capable by dissociation in accordance with the law of mass action 'of maintaining the acidity of the fixing bath within the desired limits. The weaker acids, that is, ones which are but partially dissociated in solution, such as the organic acids, for instance acetic acid, citric acid, formic acid and others, have been found to be admirably adapted for use in such a bath. As the buffer material, I have found particularly useful the salts of the alkali metals (including ammonium) of the acid used. Thus,'acetic acid and sodium acetate, or citric acid and sodium citrate or any weak acid such as those mentioned above with alkali metal salts of such acids may be used together with satisfactory results. The buffer ma erial, by

which, as previously stated, I mean any sub-- baths such as I have described.- Boric acid can also be used, as an ingredient of such baths in combination with acids of the type of those just mentioned, for example, acetic acid.

The following formula is a typical one embodying my invention:

Sodium thiosulfate 300 grams Sodium sulfite 50 grams Acetic acid (glacial) 2.4 grams sodium acetate 10.5 grams water-to 1 liter I: desired there may be added a suitable hard-- ener such as potash alum, 60 grams.

Another formula is:

,Sodium thiosultate grams Sodium sulfite-' 20 grams Citric a 4 grams Sodium citrate 8 grams Water 600 cc.

to which may be added a suitable hardener.

While I have endeavored to point out the principles of invention and to give preferred examples thereof, it is to be understood that there is possible a wide range of equivalents, the-use of tained precisely constant during the entire life of the bath, but that it is maintained within certain reasonably narrow limits, at a value such that the bath is useful.v

What I claim is: I

1. A photographic fixing bath comprising in aqueous solution, sodium thiosulfate,'an aliphatic organic acid, and a salt 01' an aliphatic organic acid in addition to salts formed in the'solution by interaction of any of its ingredients, said salt being capable of maintaining, and present in sufficient concentration as to maintain the hydrogen ion concentration of the bath at a predetermined value by dissociation for the useful life of the bath.

2. A photographic fixing bath comprising inaqueous solution, sodium thiosulfate,sodium sulfite, an aliphatic organic acid and a salt of an aliphatic organic acid in addition to salts formed in the solution by inter-action of any of its ingredients, said salt being capable of maintaining,

, and present in suflicient concentration as tomaintain the hydrogen ion concentration of the bath at a predetermined strength by dissociation for the useful life of the bath.

a. a phttogm mc fixing bath comprising in aqueous solution, sodium thiosuliate, sodium sulfite, acetic acid; and a salt of an aliphatic organic acid in addition to that formed by inter-action between the sodium salt being capable of maintaining, and present in sufiicient concentration as to maintain the hydrogen ion concentration of the bath at a predetermined strength by dissociation for the useful life of the bath. v 7

4. A photographic fixing bath comprising in aqueous solution, sodium thiosulfate, sodium sul- ;fite, acetic acid, and an alkali metal salt of said acid in addition to that formed by interaction between the sodium s'ulfite and the acetic acid, said salt being capable of maintaining, and present in sufiicient concentration as to maintain the hydrogen ion concentration of the bath between pH=4 and pH=6.5 by dissociation for the useful life of the bath. a

5. An acid hardening fixing bath comprising in aqueous solution, sodium thiosulfate, sodium sulfite, potassium alum, an aliphatic organic acid, and a salt of an aliphatic organic acid in addition to salts formed by interaction between ingredients of the bath, said salt being present in sufilcient concentration to maintain the hydrogen ion concentration of thebath at a predetermined value for the useful life of the bath.

6. An acid hardening fixing bath comprising in aqueous solution, sodium thiosulfate, sodium sulfite, potassium alum, an aliphatic organic acid, and a salt of an aliphatic organic acid in addition to salts formed by interaction between ingredients of the bath, said salt being capable oi maintaining, and present in sufllcient concentration as to maintain the hydrogen ion concentration of the bath within the range pH=4 and pH=V6'.5 by dissociation for the useful life of the bath.

FELIX A. ELLIO'I'I.

sulfite and the acetic acid, said 

